1. Field of the Invention
The invention relates to a method for manufacturing a silicon carbide single crystal by the solution method.
2. Description of the Related Art
Because silicon carbide (SiC) has a larger energy band gap than silicon (Si), various types of manufacturing art have been proposed for manufacturing high-quality SiC single crystals suitable as semiconductor materials. While a diverse range of methods of manufacturing SiC single crystals have been tried, the sublimation method is the method in general use at present. Although the sublimation method features a high growth rate, it has the disadvantages of defects such as micropipes and disturbances of the crystal structure such as multiple crystal structures. In contrast, the solution method, although it has a relatively slow growth rate, is gaining attention because it does not have these disadvantages.
In the solution method, a temperature gradient is created in molten silicon in a graphite crucible, in which the temperature decreases from within toward the surface of the molten silicon. Carbon that is dissolved in the molten silicon rises from the high-temperature region near the bottom of the graphite crucible, due mainly to convection of the molten fluid, and becomes supersaturated when it reaches the low-temperature part in the vicinity of the surface of the molten silicon. An SiC seed crystal is held on the end of a graphite rod immediate below the surface of the molten liquid, the supersaturated carbon crystallizing as an SiC single crystal by epitaxial growth on the SiC seed crystal.
In the solution method, however, there are cases in which even a slight change in growth conditions, such as the carbon concentration in the molten liquid and the liquid temperature and the like at the crystal growth surface prevents uniform single crystal with a planar growth surface, and polycrystallization, in which a plurality of separate growth hills are generated, tends to occur.
In practical use, it is desirable to increase the growth rate as much as possible, and increasing the temperature gradient is an effective method to achieve this. However, when the temperature gradient is excessive, excessive solid carbon at numerous locations in contact with the molten solution simultaneously generates multiple crystal cores, each growing to form polycrystallization. Additionally, even if the temperature gradient is optimized, the growth conditions at the crystal growth surface constantly change with time. The cause of this is generation of polycrystals in the vicinity of the crystal growth surface and at the surface of the crystal seed holding rod or deformation of the graphite crucible caused by a carbon solid solution. This prevents maintenance of single crystal growth with a planar growth surface, resulting in polycrystallization.
In addition to controlling the temperature gradient, various proposals have been made as methods of increasing the single crystal growth rate. One of the basic parameters controlling the rate of growth of an SiC single crystal is the rate of supply of Si and C to the growth surface. Because of the presence of a large amount of Si as a solvent, the carbon concentration dissolved in the molten silicon is a rate-determining factor.
Japanese Patent Application Publication No. JP-A-2000-264790 describes the increase of the growth of an SiC single crystal by adding a transition metal into the molten silicon to increase the carbon solubility. Based on this art, Japanese Patent Application Publication No. JP-A-2002-356397 describes the addition to the molten silicon, in addition to the transition metal, a rare earth metal, and simultaneously supplying a hydrocarbon gas to the molten liquid to increase the carbon concentration in the molten silicon, resulting in high-speed growth of the SiC single crystal. That the addition of a rare earth metal increases the solubility of carbon in the molten silicon is known from Dieter H. Hoffmann, et al., “Prospects of the Use of Liquid Phase Techniques for the Growth of Bulk Silicon Carbide Crystals,” Material Science and Engineering, B61-62 (1999), p. 29-39. Additionally, Japanese Patent Application Publication No. JP-A-2004-2173 describes art for achieving high-speed growth of SiC single crystals by the addition of Mn or Ti to the molten silicon.
However, while all of the above related art is directed at achieving high-speed growth, it does not address the issue of achieving a planar growth surface. Under growth conditions that achieve a high growth rate, because SiC single crystals tend to generate multiple cores or polycrystallization at multiple locations on the growth surface, maintenance of stable, planar growth over the entire period of crystal growth is difficult. To obtain a high-quality SiC single crystal suitable for practical use, it is thus essential to achieve both high growth rate and planar growth.
Upon experimenting using the method of manufacturing described in Japanese Patent Application Publication No. JP-A-2004-2173, maintenance of planar growth required that the growth rate be maintained at 50 μm/h or lower. Although the addition of Ti increases the amount of dissolved carbon, it was not possible to maintain planar growth using the growth rate noted in this reference.
In growing an SiC single crystal using the solution method, because of time variation of the atmosphere in the vicinity of the growth boundary, although a uniform planar growth layer is obtained at the beginning of growth, multi-core structure becomes prominent as the growth layer increases in thickness. The resulting roughness of the growth surface prevents the planar growth.
Although with an increase in the amount of dissolved carbon, growth is possible with even a small temperature gradient, the large amount of carbon promotes a multi-core structure, resulting in a roughened growth surface, preventing planar growth.
The foregoing described art for manufacturing a single crystal does not achieve both growth rate and planar growth at the same time.